The present invention relates to a boat drive unit, comprising a propeller drive unit fixedly arranged to the outside of a boat hull, having an at least essentially vertical drive shaft, which, via an angle gearing enclosed in an underwater housing, drives at least an essentially horizontal propeller shaft with a tractive propeller arranged on the forward facing side of the underwater housing, a rudder mounted in a wing-profile-like portion of the underwater housing for pivoting about a vertical axis aft of the propeller and a drive unit arranged inside the hull, with which the vertical drive shaft is drivably connected.
A drive unit of this type is shown and described in e.g. EP 0 269 272. The advantage of arranging the propeller to be tractive, i.e. pulling, instead of pushing is that it will then work in undisturbed water, since the underwater housing of the drive unit will lie behind the propeller. This makes it also possible to make the rudder as a wing flap-like extension of an underwater housing with a wing profile. The result will be a propeller drive with high propeller efficiency. The installation will be simplified and the installation weight will be lower than that of a steerable drive unit with a pushing propeller.
The steering capability will be good at most speeds, even with a rudder blade, the area of which is less than half of the wing profile of the underwater housing. What one might suspect is that the steering capability would be somewhat poorer than that of a steerable drive at lower speeds but comparable at higher speeds. The result in practice, however, can be just the opposite. At speeds of circa 30 knots and upwards, at rudder angles greater than a certain angle, cavitation can occur, leading in the worst case to the rudder completely losing its grip and thus the loss of steering capability.
The purpose of the present invention is in general to achieve a drive unit of the type described by way of introduction with a small rudder which provides high steering forces at all speeds and, above all, eliminates the risk of the rudder losing its grip due to cavitation at high speeds.
This is achieved according to the invention by virtue of the fact that the rudder comprises a first rudder blade, which is mounted in the underwater housing for pivoting about a first vertical pivot axes, and a second rudder blade, which is mounted in an aft-facing edge of the first rudder blade for pivoting relative to the first rudder blade about a second vertical pivot axis.
By arranging the first rudder blade as a main rudder and the second rudder blade as a wing flap on the main rudder, the centre of pressure of the underwater housing and the lateral plane formed by the first and second rudder blades will be moved aft, which creates a larger steering force than if the two rudder blades were to be replaced by a single rudder blade having the same surface area. It has been shown that if the second rudder blade has a surface which is not greater than circa 30-40% of the surface of the first rudder blade, and the steering angle of the second rudder blade relative to the first rudder blade is equal to the steering angle of the first rudder blade relative to the underwater housing, a steering angle of circa 10xc2x0 for each rudder blade is sufficient to achieve the same steering force as a 20xc2x0 steering angle of a drive unit with a single rudder blade. The smaller the steering angle is, the less will be the risk of cavitation when turning at high speeds, in order to achieve a pressure differential which is as great as possible between the two sides of the lateral plane when turning, the gap between the underwater body of the drive unit and the pivotable rudder must be as small as possible. The smaller the required maximum rudder steering angle, the simpler it will be to achieve a smooth and gapless transition between the aft-edge of the wing-profile underwater body and the forward edge of the rudder. There should preferably be no gap at all between the underwater housing and the first rudder blade and between the first and second rudder blades. In a preferred embodiment, sealing strips are used to completely seal the gaps.
In order to gain additional advantage from the fact that a tractive propeller on a propeller drive unit works in undisturbed water in front of the underwater body of the drive unit and thus has higher propeller efficiency than a pushing propeller, in a preferred embodiment of the drive unit according to the invention, the angle gear unit is arranged to counter-rotationally drive two concentric, essentially horizontal propeller shafts each having an individual propeller. In this manner the total efficiency of the drive unit can be further increased. Drive units of this type are particularly suited to fast boats of a size exceeding 40 feet, where double propeller arrangements provide high performance at the same time as the rudder arrangement with a main rudder and a rudder similar to a wing flap assures good maneuverability at all speeds.
Another possibility, provided by a drive unit with a tractive propeller, is placement of the exhaust port in the aft-edge of the underwater housing, to utilize the ejector effect which the water flowing by exerts on the exiting exhaust, in the same manner as when the exhaust exits through the propeller hub. When the exhaust exits through the aft-edge of the underwater housing instead of through the hub, the hub diameter can be reduced, which is to advantage in several respects. On the one hand, the mass and the mass forces are reduced, and on the other hand, the space required under the hull bottom is reduced, which means that the drive unit leg can be made shorter and thus lighter than for propellers with exhaust exit in the hub.
It has proved to be hydrodynamically advantageous, even with a drive unit with the above described combination of a main rudder and a rudder resembling a wing flap, to arrange a double propeller combination, known per se, with an aft-propeller, which, at least at higher speeds, cavitates when the forward propeller does not cavitate.